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THE TRANSPORTATION OF GASES                        175
tests have been made by discharging the compressed air through orifices and calculating the quantity of free air delivered per minute by Fliegner's formula.
Fleigner's formula may be stated as follows: G = (0.53JuP)/(-\/T)> where G — flow in pounds per second, A — area of orifice in square inches, P = absolute pressure (in pound per square inch) of air behind the orifice, and T = absolute temperature (degree Fahrenheit) of the air behind the orifice. The weight of 1 cu. ft. of air is found by the following formula: W = 1.325B/T, where W = weight of 1 cu. ft. of air, B = barometer reading in inches of mercury, and T = absolute temperature (degree Fahrenheit) at the compressor intake.
The delivery of cubic feet of free air per minute then equals G X (60/TF).
Efficiency.—In practice it is found that compressors with mechanically operated rotary inlet valves show volumetric efficiency varying from 91 per cent at 100 r.p.m. to 88 per cent at 188 r.p.m. Piston inlet machines at 100 r.p.m. give 88 per cent efficiency, and at 188 r.p.m., 79 per cent.
Centrifugal Compressors.—Centrifugal compressors differ from centrifugal pumps (see p. 114) only in handling gases instead of liquids, and are similarly classified as regards the number of inlets per impeller, the direction of the impeller tips at impeller exit, and the number of stages employed. They are further classified into low-pressure (1 to 5 Ib. per square inch) and high-pressure (above 5 Ib.) compressors, and also into the radial-inlet type and the axial-inlet type according as the gas enters at right angles to the shaft or in a direction parallel to the shaft.
Centrifugal compressors for pressures below 1 Ib. per square inch are generally known as blowers or centrifugal fans (see p. 149); in these the kinetic energy of the gas at the impeller exit is usually allowed to dissipate itself in eddies. For air pressures of 5 Ib. per square inch and under, a single impeller is generally sufficient. For comparatively light gases, however, a pressure of 5 Ib. may require two or more impellers in series, or a multi-stage compressor. Such a compressor is also frequently spoken of as a high-pressure gas compressor. For quantities of gas of 10,000 cu. ft. per minute and over, compressors of the radial-inlet type require shrouds or reinforcing rings at the inner ends of the impeller blades to prevent the wide blades from crumpling at the inlet under the action of centrifugal stresses. Impellers of the axial-inlet type are not subject to such crumpling, and are therefore generally used when large volumes are handled. As to single-inlet and double-inlet impellers, see remarks on page 161. The radial-discharge impeller is the one best adapted for high peripheral speeds, and is the type most commonly used. Both the backward-discharge and the forward-discharge impellers require shrouds at their outer peripheries. The former are frequently resorted to when large gas quantities (requiring a large impeller inlet) are to be raised to a comparatively low pressure with a direct-connected high-revolu-tion-per-minute driver; the latter are but rarely used. Multi-stage compressors are usually provided with special means for cooling the gas during its passage through each impeller and from stage to stage, and also for preventing leakage from stage to stage and to the atmosphere.
The centrifugal compressor occupies comparatively little room for its output; direct-connected to an electric motor or to a steam turbine, it forms a very compact unit. Besides its bearings it has no rubbing or wearing parts; it contains no moving valves or springs; and it requires a minimum of attendance and oiling. It is also fairly free from vibrations and requires comparatively light foundations. At constant revolutions per minute it will maintain approximately constant pressure for widely varying quantities of gas, which makes it very desirable for general power transmission. If no gas is required temporarily the discharge pipe may be shut off without